get tests passing on Windows

[bcoe]

This wasn't as much work as I'd feared, fixes #81 \o/

Here's a break down of the changes I've made:

• I've added the appveyor.yml that @sindresorhus recommended.
• I switched to using rimraf for deleting left over test data, since it's cross-platform.
• windows doesn't (or at least signal-exit on Windows) doesn't support process.kill(), I've disabled the two tests that require process.kill using the is-windows module.
  • I've also made the spawn tests not rely on scripts that call process.kill.
• I was running into issues with path.sep being \ on Windows and / on *nix.
  • nyc and Istanbul have paths that are all over the place on Windows (sometimes using one separator, sometimes another).
  • rather than try to unravel this mess (since it doesn't break anything but tests) I've written the helper module any-path: https://www.npmjs.com/package/any-path

This work would not have been possible if it wasn't for the hard work of @jamestalmage @novemberborn , refactoring and fixing up tests.

Thank you everyone for all the hard work on this module recently \o/

Reviewers: @jamestalmage @novemberborn

[novemberborn]

Nice!

[sindresorhus]